The process technology for the manufacture of polypropylene (PP) has evolved with improvement in catalyst technology, from complex slurry processes using an inert hydrocarbon diluent, to simpler bulk processes using liquid propylene diluent, to even more simplified gas phase processes.
Gas phase reactor processes widely known and well described in the art include those based on continuously stirred tank reactor and fluid bed technologies. Examples of such reactor systems are described in U.S. Pat. Nos. 3,957,448; 3,965,083; 3,971,786; 3,970,611; 4,129,701; 4,101,289; 3,652,527; and 4,003,712, all incorporated herein by reference. Typical gas-phase olefin polymerization reactor systems comprise at least one reactor vessel to which olefin monomer and catalyst components can be added and which contain an agitated bed of forming polymer particles. Generally, catalyst components are added together or separately through one or more valve-controlled ports in the single or first reactor vessel. Olefin monomer may be provided to the reactor through a recycle gas system in which unreacted monomer removed as off-gas and fresh feed monomer are mixed and injected into the reactor vessel. Polymerization will be carried out under conditions that exclude oxygen, water, and other materials that act as catalyst poisons. Polymer molecular weights are controlled through use of additives such as hydrogen in a manner well known to persons of skill in the art.
The Amoco Gas Phase Process may be generally characterized as being conducted using two horizontal, stirred-bed, gas phase reactors in series. The plug-flow reactors employ an interlock system separating the first stage homopolymer reactor from the second stage copolymer reactor. The process provides an inherently narrow residence time distribution coupled with optimized stirring, minimizing temperature variations and resulting in greater product consistency. The Amoco process is disclosed generally in “Polypropylene Handbook” pp. 297–298, Hanser Publications, NY (1996), and is more fully described in U.S. Pat. No. 3,957,448 and in “Simplified Gas-Phase Polypropylene Process Technology” presented in Petrochemical Review March, 1993. The teachings of these publications and the patent are hereby incorporated in their entirety by reference.
Rubber-modified polypropylene resins are prepared commercially for the most part by post-blending separately produced homopolymer and copolymer resins in a compounding operation. In-reactor processes wherein homopolymer formed from the first monomer in a first reactor is subsequently reacted with the second monomer in a second reactor have also been disclosed and described in the art. Gas phase reactor processes such as are described in Hydrocarbon Processing 74 pp. 140–142 are disclosed to be useful for the production of impact PP resins. The two-stage horizontal gas phase reactor described in Polyolefins VI SPE RETEC, Houston, Tex. (1991), page 68 has also been employed in the production of impact polypropylene. Processes for use in the manufacture of copolyolefins have been further described in Petrochemical Review, March, 1993, in U.S. Pat. No. 3,957,448 and in Chemical Engineering Science Vol. 47, no. 9-11 (1992) pp. 2591–2596.
The polymerization catalysts conventionally employed in these processes have generally been Ziegler-Natta type catalysts. For example, the Amoco gas phase process is disclosed in the art to employ fourth generation supported catalysts consisting of three components: a proprietary solid CD catalyst, a trialkylaluminum activator or cocatalyst, and an external modifier or donor. Separately, the catalyst components are inactive. Hence the CD catalyst and activator may be suspended in propylene and fed to the reactor as separate streams without initiating polymer formation in the feed lines.
Recently there has been developed a practical catalyst technology based on metallocene compounds, termed sixth generation catalysts by E. Albizzati et al. in “Polypropylene Handbook”. Metallocene catalysts, more particularly described as Group 4 or 5 metallocenes, are soluble organic complexes that result from the reaction of biscyclopentadienyl transition metal complexes (metallocenes) with a cocatalyst, generally an aluminum compound. Most metallocene catalysts employed for propylene polymerization are zirconium-based, and the most widely used cocatalyst is methylaluminoxane (MAO), derived from trimethylaluminum (TMA). Other metallocene catalyst systems disclosed in the art include combinations of metallocene dialkyls with boron compounds, further including trialkylaluminum compounds.
Supported metallocene-based catalyst systems, which may be more particularly described as fully active, metallocene-based catalyst systems immobilized on a particulate carrier having narrow size distribution such as a finely divided silica, alumina, MgCl2, zeolite or the like, are also known. Solution and bulk processes for ethylene and propylene polymerization employing supported metallocene-based catalysts have been disclosed and are well described in the art.
Metallocene catalysts are difficult to employ directly in conventional polymerization processes, and particularly in gas phase processes where the catalyst system will be dispersed in a hydrocarbon or in monomer and metered into the reactor through feed lines. Supported metallocene catalysts are optimally active when preactivated, i.e. combined with the cocatalyst component prior to being introduced into the reactor. Dispersing such catalysts in the olefin monomer stream for direct feed to the reactor system results in polymer formation and causes severe plugging of the feedlines. Moreover, polymerization proceeds before the catalyst system is dispersed fully and uniformly through the polymer bed in the reactor, resulting in highly active hot spots that promote the formation of lumps and plating out. The reactor rapidly becomes fouled, reducing catalyst yields and requiring frequent shutdowns to clean the reactor.
Inert gases, hydrocarbons and the like have been employed as diluents and as carriers for use with Ziegler-Nafta catalysts. These methods have had some success when employed with soluble metallocene catalysts in solution and bulk polymerization systems. In gas phase processes employing continuously stirred tank reactor and fluid bed technologies, the use of such diluents and carriers for feeding supported metallocene catalyst systems to the reactor with the olefin stream has generally not been successful. Although the problem of plugging may be avoided by dispersing the supported catalyst in an inert hydrocarbon such as propane and separately metering the mixture to the reactor, it is difficult to adequately disperse the catalyst through the reactor polymer bed rapidly enough to avoid forming lumps and strings.
Temporarily reducing the activity of metallocene catalysts has been described in the art. For example, adding a dialkyborane or dialkylaluminum to the reactor during a polymerization to temporarily retard the activity of metallocene catalysts has been disclosed as a method for process control. However, catalyst activity is only partially retarded by such treatment. Catalysts directly treated with a dialkyborane or dialkylaluminum retain sufficient activity to initiate polymerization when dispersed in the monomer feed stream. Moreover, the recovery period is very brief, too brief to allow the catalyst system to be adequately dispersed in a stirred reactor gas phase reactor bed before the catalyst recovers and polymerization proceeds.
It is known that metallocene catalysts are deactivated by Lewis acids. Reactivating a Lewis acid-treated catalyst after it is dispersed in the reactor bed requires adding excess MAO, which is difficult to disperse because of its low volatility. Separately adding an alkali metal alkyl or alkaline earth metal alkyl and a fully active, supported metallocene catalyst to a reactor before contacting with monomer has been disclosed to be useful for avoiding lumps and wall formations in the suspension polymerization of ethylene polymers and copolymers. The use of Lewis bases to retard or terminate a metallocene catalyzed polymerization as a means for process control is also disclosed in the art. Restarting the polymerization, accomplished by adding excess MAO, may require adding as much MAO as was employed in the initial preparation of the catalyst. Due to poor volatility, dispersing the MAO uniformly through the reactor bed is difficult, and the polymerization activity after restart may be substantially reduced. Moreover, many Lewis base compounds are irreversible catalyst poisons. In a continuous process such poisons will accumulate in the reactor over time, requiring that the process be stopped while the reactor is cleaned.
Thus, there does not appear to be available a method for temporarily and reversibly passivating metallocene catalysts whereby catalyst activity becomes reduced to a level that will allow feeding the catalyst to the reactor in contact with olefin monomer and adequately dispersing the catalyst in the reactor polymer bed prior to reactivating.